Laboratory tests have shown that certain tumors and cancerous tissue can be effectively treated by heat in excess of body temperature. Ultrasonic energy is presently used to create localized heating within the body for the treatment of many types of tumors. The temperature to which the tumors must be heated to effectively treat the cells is in the range of 43.degree. -48.degree. C. (109.degree. -118.degree. F.), but may vary somewhat, according to various factors.
A consideration in the use of ultrasonic devices to create localized heating is that the linear absorption coefficient of ultrasonic energy of bone is very large compared to that of a tumor being treated or surrounding muscle tissue. There is a 26% power absorption per millimeter of ultrasonic energy in bone at 1 MHz, increasing proportionately with the square of the frequency up to 2 MHz. There is only to 2.3% power absorption per millimeter of ultrasonic energy in muscle, with less frequency dependency. This can result in the bone becoming hotter than the tumor with moderate to severe bone pain. This may occur even if the bone receives less energy than the tumor or if the bone underlies the tissue which is being treated.
An ultrasonic hyperthermia treatment device is described in the 1985 Ultrasonics Symposium proceedings, pp. 942-948, IEEE, entitled "A Large Aperture Ultrasonic Array System for Hyperthermia Treatment of Deep-Seated Tumors," by Seppi et al. It has an array of transducers, each having a conical lens, mounted in a water bath. Each transducer is independently controllable in power, phase, and mechanical orientation. This device is bulky, complex, expensive, and difficult to operate.
A hyperthermia system using a transducer coupled with an ultrasonic imaging system is disclosed in an article titled "A Therapeutic Ultrasound System Incorporating Real-Time Ultrasonic Scanning," by Lizzi et al., the 1986 Ultrasonics Symposium Proceedings, IEEE, 981-984.
These devices and methods of operating them have numerous disadvantages. A transducer with or without a lens often creates an energy distribution pattern having multiple local minima and maxima in the near field. The depth, spatial location and strength of the minima and maxima vary with transducers, frequencies and lenses.
The near field linear intensity profile of a sample heating transducer with no lens is illustrated in FIGS. 1a-1d. These are lateral profiles measured 2 cm from the face of a 6 cm diameter unfocused transducer at different frequencies. The intensity profile at each frequency has minima and maxima that are non-uniformly distributed along the face of the transducer.
At 0.75 MHz the transducer energy profile has a minimum value at the center with two peaks on either side of the center as shown in FIG. 1a. At 1.00 MHz this transducer has an energy profile with a peak value in the center with various maxima and minima along the diameter as shown in FIG. 1b. At 1.20 MHz a peak value is at the far right hand side with a second peak in the center and a third, slightly lower peak at the far left hand side, as shown in FIG. 1c. At 1.35 MHz a more even distribution of maxima and minima is produced by the transducer but major variations in the intensity profile exist. These maxima and minima result in hot and cold spots in a region being treated with ultrasonic energy. Even small changes in frequency can significantly alter the intensity profile. Similarly, different transducers have different intensity profiles.
When high intensity energy, such as the peak in FIG. 1b, is repeatedly applied to tissue, the tissue heats excessively while other tissues received much less energy and are not heated. Similarly, an energy peak near the edge, such as in FIG. 1c, may inadvertently fall on healthy tissue or bone and cause undesirable heating and pain while the tumor is not sufficiently heated.
In some systems an array of transducers are used. Interactions between the sound fields of each transducer creates a complex linear intensity profile with additional maxima and minima created by the interaction. This results in greater variations in hot and cold spots and the location of each is more difficult to predict.
To aid in the elimination of undesirable interaction of sound fields from an array of transducers, the energy may be modulated in magnitude, phase or frequency. A second approach to prevent heating of localized regions of bone or tissue is to continuously move the transducer assembly in both a linear and pivoting manner (i.e. wobbling) so that the maxima and minima do not rest on any localized region for a significant period of time. Operator skill is critical in the manual manipulation of the transducer assembly. A less skilled operator may over heat bone without adequately treating the tumor.